So I have a 42VDC supply that I want to feed into a linear regulator. But almost all linear regulators want 40V absolute maximum, and the ones that have higher maximums are not reliably stocked anywhere. So I need to drop that down by at least 5V or so. The good news is that I'm only pulling around 70mA on this, so whatever drops it doesn't have to dissipate much power. It occured to me to use a normal shunt zener, but that will end up dissipating extra power through the zener, which is just wasteful. It also occured to me to put a bunch of diodes in series and use their Vf drops, but that would require quite a few diodes. So why not use a 5V zener (reverse biased) in *series* with the load? It will never dissipate extra power, only what is required by the load.
Except then it occured to me that zeners have some sort of minimum current to keep them stable, which seems to be around 0.25mA. What would happen if my load started drawing <1mA, or was just disconnected entirely (leaving only the quiescent current of the linear regulator)? I *think*, from looking at the V vs I charts, that the zener would start leaking current with very little voltage drop, thereby subjecting the linear regulator to higher than its rated voltage. Though this would be only at a very small current, so maybe it wouldn't hurt? I dunno, but I suspect that operating it in that range 24/7 for a while would do some damage no matter what. So the idea I had is to put a resistor in parallel with the load sized to draw the zener's minimum current in all cases. That gets me back to wasting current -- but only ~0.25mA, which is orders of magnitude less than I'd waste in a shunt arrangement. (That works out to ~9mW). In terms of total power dissipation, this would be roughly the same as a zener plus an emitter follower (zener carries minimum zener current, transistor carries load current), but without the extra parts. Any thoughts? Am I crazy? TIA, -- Randall _______________________________________________ geda-user mailing list [email protected] http://www.seul.org/cgi-bin/mailman/listinfo/geda-user
